Identifying a number of objects suspended in a medium is typically accomplished via particle counters employing a microscope to acquire an image of a section from the medium. Assuming that the particles remain in a homogeneous suspended state, a counted total of each of the objects within the section may be used to project a value representative of the density of objects in the medium. Automated particle sizing and counting on a microscopic scale began around 1954 with a Coulter Counter, which employed electrical sensing zone techniques. In particular, the particles to be measured were dispersed in an electrolyte solution and passed through a tube having a narrow aperture with electrodes on either side. The narrow aperture restricts the particles so that only a single particle passes through at one time through an electric field. As the particles pass through the aperture, a resistance is measured, which is related to a corresponding, particle size.
Techniques developed after the Coulter Counter evolved to reduce the time required to measure particles and the efficiency at which particles were measured via diffraction and/or scattering techniques using light sources, such as single wavelength lasers. Generally speaking, diffraction techniques identify characteristic signatures of light after a particle influences the incident light. Such characteristic signatures may be derived from ring-shaped intensity patterns indicative of particle size, in which closely situated rings identify corresponding particles having a relatively larger size and widely situated rings identify corresponding particles having a relatively smaller size. The diffraction techniques permitted, an improved ability to measure particles having smaller sizes than were capable via the Coulter Counter. Diffraction measurements allowed measurements down to particles having a 20 nano-meter (nm) diameter, but do not collect all of the scattered light, thereby limiting the resolution and sensitivity. Scattering techniques typically use a single detector, multiple detectors or an array of detectors, but only collect a fraction of the scattered light, which limit resulting resolution and/or sensitivity.
Scattering techniques to determine a size and/or distribution of particles include laser diffraction, dynamic light scattering, angle dependent scattering, in which a fixed-wavelength laser is directed on a solution of particles and a single detector, multiple detectors or an array of detectors is arranged to collect light scattered from the solution. The techniques that analyze scattered light include dynamic light scattering, angular dependent scattering, laser diffraction, and photon cross correlation spectroscopy. Such techniques employ a single detector, multiple detectors, or an array of detectors and may provide information indicative of the strength and distribution of the light to derive particle size and/or distribution information. Accordingly, the diffraction/light scattering techniques miss a substantial fraction of the total possible paths of flight that are scattered by one or more particles in the sample.